Scientists from the NTU School of Chemistry, Chemical Engineering and Biotechnology (CCEB), have developed a sophisticated analytical platform, one that has led to the discovery of an alternate signalling pathway between our immune system and our gut microbiome, and the identification of the bacterial fragments that trigger that pathway.
Their efforts help to expand our current understanding of our immune systems vis-à-vis our gut microbiome, an area of study that we are only beginning to unravel.
From left: Li Chenyu, Nanyang Assistant Professor Qiao Yuan, Christopher Adamson
Bacteria in our gut
The human body is home to trillions of microorganisms, which include viruses, bacteria and fungi. They make up what has been termed the “human microbiome,” with the largest and most diverse site situated in our digestive system. Scientists estimate that there may be more than 5,000 species within our digestive system alone.
These microorganisms form a complex ecosystem in our gut and are very important to our overall health. They help us to digest our food, absorb nutrients, protect against harmful germs and bacteria and support our immune system.
However, our understanding of the relationship between our gut microbiome and our bodies is still in its infancy, and there is much that we still need to make sense of. One such area is how our gut microbiome affects our immune system and what biological pathways are involved.
Peptidoglycan fragments
The bacteria in our gut have cell walls, which are made from a molecule known as peptidoglycan. Peptidoglycan is made up of a combination of long carbohydrate chains and short amino acid chains, giving it a tough mesh-like characteristic that is crucial to forming the bacterial cell walls.
When the bacteria in our intestines grow, divide, or are attacked by enzymes, their cell walls weaken and fragment. These peptidoglycan fragments are released into our gut environment, where they can cross the intestinal lining and enter our body. They act as important signalling molecules between our gut and our body’s immune system when they trigger special sensors in our immune system, known as pattern recognition receptors (PRRs). PRRs can detect microbe associated molecular patterns (MAMPs), essentially molecules found on bacteria and viruses, including peptidoglycan fragments. Upon detection, our immune system responds with reactions like inflammation.
When things go wrong with the bacteria in our gut environment and signalling pathways between the MAMPs and PRRs, it can lead to intestinal diseases like inflammatory bowel disease. To treat these diseases more effectively, scientists need to have a greater understanding of the biological pathways through which our bodies recognise and react to these gut bacteria signals.
The NODs
Existing research into the pathways between bacterial fragments and our immune system has focused on the study of two PRRs, NOD-1 and NOD-2. NOD, or nucleotide binding oligomerisation domain-containing proteins, are found in the gel-like material within cells known as cytoplasm, and both NODs recognise peptidoglycan fragments. NOD-1 recognises iE-DAP, a dipeptide – a molecule made up of two amino acids joined together via a peptide bond – while NOD-2 recognises MDP or muramyl dipeptide.
However, there are numerous other peptidoglycan fragments besides iE-DAP and MDP that are missed by current research methods. These other fragments appear to use alternate pathways to trigger immune responses. For instance, the team from NTU previously identified a fragment from Bifidobacterium, a probiotic, which does not engage the NODs. This indicates that there are plenty of other signalling pathways and peptidoglycan fragments to be discovered.
A more comprehensive platform and new insights
The team, led by Nanyang Assistant Professor Qiao Yuan, developed a liquid chromatography, high-resolution tandem mass spectrometry (LC-HRMS/MS) platform that can more comprehensively identify and measure the amount of peptidoglycan fragments in a host’s gut.
The platform works by separating mixtures based on their chemical properties, then measures the exact mass of each separated molecule with very high accuracy. It then fragments the molecule further and analyses the resulting pieces to determine the structure, and hence identity, of the original molecule.
With their new platform, the team expected to find MDP fragments that activate NOD-2. However, they discovered that there were larger amounts of a disaccharide – a molecule made up of two simple sugars – known as GlcNAc-MurNAc, or GM for short, ubiquitously found in the gut of every individual.
GM was able to activate mild-stimulatory effects, but it does not trigger the NODs. Instead, it triggers another PRR called the toll-like receptor (TLR4). In subsequent experiments to find out the immune responses that it can trigger, GM was introduced into an in vivo model that had chemically induced inflammation in the colon. This resulted in a significantly reduced inflammation of the colon.
The team’s experiment confirms that there are not only other peptidoglycan fragments involved in the signalling process between our gut bacteria and our immune system, but there are also other signalling pathways involved. This broadens our understanding beyond just the NOD receptors and the iE-DAP and MDP fragments.
What does the future hold?
The team is optimistic that their discoveries may lead to the development of new treatment options for intestinal inflammations. They are currently working on modifying the structure of the GM disaccharide in order to discover other similarly potent disaccharides that can treat other inflammatory intestinal diseases or infections, subsequently leading to the development of GM-based postbiotics or probiotics. They have also filed a Singapore and a US patent based on their findings, which will enable them to further develop upon their research.
To find out more about their incredible work to treat gut diseases, click here.